Area and computation time are considered to be important measures with which VLSI circuits are evaluated. In this paper, the area-time complexity for nontrivial n-input m-output Boolean functions, such as a decoder and an encoder, is studied with a model similar to Brent-Kung's model. A lower bound on area-time-product (ATaaa.=1) for these functions is shown: for example, AT<sup>a</sup>= ?(2<sup>n